So yes, yes you can indeed put the hearts inside the envelopes!⁠

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Though to be fair, if you knit an Absolutely Massive Heart and a Very Very Tiny Envelope, you may run into problems of scale. But if the general question is ‘is the envelope open on the inside,’ then yes, yes it is.⁠
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Though I did have a couple of folks who were a little…surprised…that, when they put a puffy thing (the heart) inside a flat thing (the envelope), the flat thing was then less flat. So I suppose maybe I should mention that the envelope is a normal physical object, not some sort of magical portal that takes the volume out of other physical objects? ⁠
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I’m pretty sure if I could make a piece of knitting that changed the volume of the material contained within it, I’d have paid off everyone’s student loans and medical debt and retired to my own private island by now. ⁠Tell me what magical properties you’d like to imbue your knitting with and what you’d do if that worked out!⁠
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Envelope pattern is Dispatch (rav, payhip), heart pattern is Palpitation (rav, payhip). Both are 20% off with the code HEARTFELT through Valentine’s day. ⁠Yarns are Seven Sisters Arts’ heart pack.